The present invention relates to display devices. More specifically the invention relates to monochrome liquid crystal display devices for use in medical applications, such as radiographic diagnosis.
In radiographic diagnosis it has been common practice for very many years to use photographic film as a medium for recording a radiographic image. Diagnosis is performed by examining the image on the film when placed on a light box.
The film medium is being used for primary diagnosis as well as for communication between radiologist, physicians, doctors, patients and others. The film is furthermore used as an archiving medium.
Several film types have been used. A distinction between the different film types can be made on the basis of the inherent color of the film base. One can distinguish between clear base film and blue base films.
Among radiologist so-called blue base film has become a standard. In order to obtain a blue base radiographic film a blue dye has been added to the substrate of the radiographic film. (see Technik der Rxc3x6ntgendiagnostik, Hanno Poppe, Georg Thieme Verlag, Stuttgart, 1961, p. 571). Blue base film is far more frequently used than clear base film, the use of which has become rather limited.
Although the technical reasons for the claimed advantages of blue base radiographic film are far from convincing, it is a fact that the blue color of the film is considered an aid to diagnosis (see Radiographic processing, D. H. O. John, Focal Press London, page 238-239). Advantages of this so-called blue base film are described by radiologists as xe2x80x98cleanerxe2x80x99, xe2x80x98less fatiguingxe2x80x99, xe2x80x98coldxe2x80x99, xe2x80x98sharperxe2x80x99 and xe2x80x98with more contrast perceptionxe2x80x99.
With the introduction of Picture Archiving and Communication Systems (PACS) and workstation based applications such as computed radiography (CR), magnetic resonance (MR), computed tomography (CT) etc. cathode ray tube (CRT) monitors have more and more been used as a replacement for film.
In order to be able to perform diagnosis of the image displayed on the monitor, specific monitors have been developed and introduced in this market in order to meet the high standards.
Typically, these monitors are high resolution monochrome (grayscale) monitors. Pixels are not formed by addressing a red, green and blue subpixel in various intensity combinations like it is done on color monitors, but by directly writing with an electron beam on the CRT phosphor, without passing through a grid. In this way, higher resolutions can be obtained. However, a consequence is that in such a monitor only one type of phosphor is used to generate the light.
For medical applications, in most cases one of the following two phosphor types are currently being used in these monitors: the P104 and the P45 phosphor.
A P104 phosphor produces a light color, which is very close to neutral white.
A P45 phosphor produces a more blue-like light. The P45 phosphor has been used to generate an overall image look that resembles the overall image look which the radiologist is used to have when he is diagnosing radiographic images on blue-base film.
One of the latest developments in the softcopy environment, is the use of flat panel displays on the basis of liquid crystals.
The development of the technology of grayscale flat panels is leading to an increased use of the medium for diagnostic and viewing applications. The more the quality of these products is improved, the more they will replace the currently used medical grayscale CRT monitors.
The technology of grayscale flat panel display devices is based on another principle than that of CRT monitors.
In these display devices chemical crystals are applied that within a given temperature range (xe2x88x925xc2x0 C.-+65xc2x0 C.) act as anisotrope fluidum whereas above this temperature range they become a clear isotrope fluidum.
In the anistrope range the optical characteristics of these crystals can be changed by applying an electrical field.
In absence of this electrical field, the material is clear whereas at ca. 1 MV/m the material becomes turbid and does no longer transmit light.
The fact that these crystals may act as light valves for the light of an illuminating light source (occasionally provided with a light diffuser) placed behind the layer of liquid crystals is used when building display devices by means of these so-called xe2x80x98liquid crystalsxe2x80x99.
A liquid crystal display device commonly consists of two spaced glass panels in between which a thin layer of liquid crystals is applied. The glass panels are usually covered with a transparent electrode layer which may be patterned in such a way that an array of pixels is created. By energizing the electrodes the liquid crystal layer can be made locally turbid.
Two addressing systems are used to drive the display device: either a passive or an active system.
In the passive system the two electrode layers are patterned in a regular array of stripes. The stripes on one panel are perpendicular to those on the other panel.
The application of a voltage across two opposing stripes causes a change in the optical properties of the liquid crystal material situated at the crossing point of the two stripes, resulting in a change of light transmission through the energized pixels.
In an active system, which greatly improves the performance of the liquid crystal display device, each pixel has its own individual microelectronic switch. This means that such a microswitch is connected to an individual transparent pixel electrode, the planar size of which defines the size of the pixel. The microswitches are individually addressable. Three terminal or two-terminal switching elements exist. Three terminal switches are formed by thin film transistors arrayed in a matrix pattern on one glass platen which together with a glass platen carrying a transparent uniform (non-patterned) electrode layer forms a gap which is filed with the liquid crystal material. In case of a two-terminal switch such as a diode the second platen must also be patterned.
Monochrome as well as color liquid crystal display devices exist.
To impart color reproduction capability to the liquid crystal display device a color filter array element is provided on one of the two glass platens. In an active matrix display device, the color filter is usually provided on the glass platen not carrying the switching elements.
A color filter array for full color reproduction is composed of patches of three primary colors red, green and blue arranged in a given order. For contrast enhancement the color patches may be separated by black contour line patterns delineating the individual color filters.
In order to prevent loss of effective voltage over the liquid crystal material, the color filter is preferably kept out of the electrical circuit, which means that the transparent electrode is deposited on top of the color filter array element.
U.S. Pat. No. 5,128,782 discloses another embodiment of a color liquid crystal display device. The device comprises a liquid crystal display panel. Behind the panel are three flash tubes. The flash tubes are used for sequentially illuminating the panel with three primary colors. For this purpose in front of each of the flash tubes either a red or a green or a blue color filter is provided.
Color separation images of an image to be displayed are generated and applied to the display device. First the red separation image is displayed. For this purpose the red image is retrieved from memory and the transparency of pixels of successive lines of the liquid crystal display panel is controlled in accordance with the data of the red color separation image.
Next the procedure is continued for the green and the blue color separation images.
In medical diagnosis high resolution is extremely important. Another very important aspect is contrast. Monochrome liquid crystal display devices provide increased luminance which results in increased contrast and are therefore preferred over color liquid crystal display devices.
Another element which is very important to make medical diagnosis as reliable as possible is the consistency of the overall outlook of medical (radiographic) images.
A radiologist must be presented with images the overall outlook of which resembles as much as possible the outlook he is trained to see and is expected to see.
In this way it is avoided that his attention would be distracted by overall changed aspects of the image such as a different background color of the image rather than by elements that are of real importance for the diagnosis.
However, the monochrome liquid crystal display devices presently on the market produce a gray shade that is clearbase like because of the spectrum of the light source that is used in such a monochrome liquid crystal display device.
Hence, the image that is displayed on a monochrome liquid crystal display devices does not correspond with the characteristics of the image which the radiologist is used to diagnose. It does not resemble the blue base radiographic film, nor does it resemble the image on a CRT monitor employing a P45 phosphor.
This fact is generally not appreciated by the radiologists.
It is an object of the invention to provide a monochrome liquid crystal display device that is adapted for use in the field of medical diagnosis on displayed radiographic images.
Further object will become apparent from the description hereafter.
The above mentioned objects are realized by a monochrome liquid crystal display device having the specific features defined in claim 1.
A monochrome liquid crystal display device according to the present invention comprises a liquid crystal panel comprising a layer (1) of liquid crystals and a light source (6) arranged for uniformly illuminating the layer (1) of liquid crystals. A color filter (8) is provided for changing the spectrum of the light emitted by said liquid crystal panel.
Preferably the color filter has a spectrum such that the color co-ordinates of the light emitted by the liquid crystal display device correspond with the color co-ordinates of the blue base of a photographic film.
In a specific embodiment the color filter is the blue base of a photographic film.
The color filter is preferably provided between the light source and a viewer. Different positions are possible.
The characteristics of a filter for changing the spectrum of the light emitted by a monochrome liquid crystal display device can be determined as follows.
First the spectrum of the monochrome liquid crystal display device is measured without a filter being present and the color co-ordinates in a predefined color space such as 1931 CIE standard color space, are calculated. The result is referred to as color A.
Next, the spectrum of a blue base of a photographic film is measured on the same light-box. The color co-ordinates in the same color space are calculated. The result is referred to as color B.
Next a filter is calculated having a spectrum C so that color A applied with the filter C can be perceived by the human eye as color B.
Specific features for preferred embodiments of the invention are set out in the dependent claims.
The display device of the present invention is advantageous because it provides an acceptable compromise between two requirements set by the radiologists, namely (1) the need for high contrast which is nowadays provided by monochrome liquid crystal display devices and (2) the need for a displayed image that maximally resembles the overall look of the images the radiologist has been used to see on either radiographic film or on a CRT display device.
Further advantages and embodiments of the present invention will become apparent from the following description and drawings.